This invention relates to aircraft gas turbine engines that are capable of conversion from turbofan to turboshaft modes of operation and vice versa.
Considerable attention has been directed at various efforts to develop an aircraft engine that is capable of both turbofan and turboshaft operation. Commonly referred to as convertible aircraft engines, powerplants that have been conceived with this dual capability would be desirable for powering an aircraft in a vertical direction as a helicopter and alternatively for powering an aircraft in a forward direction as a typical turbofan-powered jet. The obvious advantage of dual mode operation is that the aircraft could take off and land like a helicopter but could also fly forward at relatively high speeds like a jet-powered airplane.
One example of a convertible engine concept is disclosed by W. J. Stein et al in U.S. Pat. No. 3,351,304. As evidenced by this disclosure, W. J. Stein et al conceived an aircraft powered by turbofan engines that are connected to a helicopter-type, vertical-lift rotor by a primary gear train with appropriate reduction gearing. The engine additionally has a secondary gear train, which, when actuated, motors the rotor at lower r.p.m. to reduce resistance during turbofan-powered forward-thrust operation with a manually operated clutch to selectively actuate either the primary or secondary gearing. The gearing and shifting arrangement permits the speed of the fan to be modulated through the gear ratios. The advantage is that in one phase of flight operation the engine is turning the fan at low r.p.m., thereby permitting a major part of the engine power to be delivered as a shaft horsepower to the rotor system, while at the same time restricting the thrust produced by the fan to a low level. In the alternate phase of operation, the gearing permits the fan speed to be increased while at the same time limiting the amounts of shaft horsepower diverted to the rotor system.
As might be expected, this type of system tends to be inefficient because the engine thermodynamics do not match the actual engine cycle. Also, the requirements of the gearing and clutching system are extreme and would require large and heavy equipment that is undesirable in aircraft applications.
Since the time period of the Stein et al invention, numerous technical advances have been made in the area of multi-cycle aircraft engines. Some of these advances permit variable bypass operation of a turbofan engine. This new technology has fostered innovations relating variable bypass system applications to convertible-type engines.